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BIOMECHANICS OF SPORT CONCUSSION: QUEST FOR THE ELUSIVE INJURY THRESHOLD

In the January 2011 issue of Exercise and Sport Sciences Reviews, the authors combine their recent studies investigating the biomechanical relationships of concussion with various factors such as playing position, types of play, concussive versus subconcussive impacts, location of impacts, and clinical measures of concussion. Through real-time data collection, the authors sought to determine whether an injury threshold for concussion does indeed exist.

A discussion of direct and indirect forces related to concussion, as well as the forces being a combination of linear and rotational in head injuries, is reviewed. Authors question why every direct or indirect impact does not result in an injurious episode. Research shows that even if the head does not move, kinetic energy still can be transferred through the skull, resulting in internal, potentially injurious deformations. Also, cerebrospinal fluid protects the brain within the cranium, so not all forces exceed the threshold required to cause the brain to impact the inside walls of the cranium. Athletes who experience a rotational mechanism are thought to encounter rotation of the cerebrum about the brainstem, producing shearing and tensile strains. These types of impacts are thought to result in more episodes of loss of consciousness than linear forces due to the fact that the upper brainstem and midbrain are responsible for alertness and responsiveness.

The use of in-helmet accelerometers has evolved over the years and now is being researched in athletes during normal practice and game situations. The impact data significantly vary between studies and also is higher in helmets-only and full-contact practices versus those sustained in games or scrimmages. The authors' study established that college football players experience 950 subconcussive head impacts in a given season, which is consistent with other studies in the literature. Newer research looking at youth hockey players reveals similar numbers.

The authors went on to examine whether an injury threshold exists as it relates to concussion. Prior data typically uses a force in the range of 80 to 90 g sustained over 4 ms as the level after which it is presumed a concussion would occur. Similarly, rotational accelerations in the range of 5900 rad·(s2)−1 were considered at risk for concussion. The data from this group's lab suggest that there is far from a 50% probability of sustaining a concussion with either an impact exceeding 80 g or 5900 rad·(s2)−1. Their findings suggest that clinicians should not expect a single impact greater than those cited necessarily to result in either immediate or delayed symptoms of concussion and/or balance or cognitive defects.

Additionally, the biomechanics of the concussions they studied demonstrated a higher propensity of top-of-the-head impacts. It also was found that the impacts occur at a wide range of magnitudes and that the clinical measures of acute symptom severity, balance, and neuropsychological function all appear to be largely independent of magnitude or location. In other words, the concussions sustained as a result of lower-end magnitudes tended to present with just as many clinical findings as those with higher magnitudes. Therefore, one cannot use magnitude and location to predict clinical recovery in concussions. The authors suggest that type of acceleration in combination with impact location may be a better determinant.

The value of biomechanics research may be best seen in the impact it can have on influencing rule changes and enforcement of existing rules. The youth hockey study the authors conducted demonstrated that head impact severity was higher with infractions versus legal collisions and that, when a player could anticipate the collision, the impact sustained was lower. In football, the head down position was more likely to result in a concussive force. Further studies are needed to help researchers interpret these findings. Bottom line: No defined injury threshold exists as it relates to concussion, and any such thresholds should be interpreted with caution. Concussion continues to be multifaceted in both cause and treatment (1).

OVERCOMING BARRIERS TO PHYSICAL ACTIVITY: HELPING YOUTH BE MORE ACTIVE

The January/February 2011 issue of ACSM's Health & Fitness Journal® provides readers with a summary of the many barriers that exist regarding physical activity in children and adolescents. According to the most recent National Health and Nutrition Examination Survey (NHANES), only the youngest children, ages 6 to 11 yr, meet physical activity guidelines. Most older youth, ages 12 to 19 yr, do not meet the guidelines. An examination of the many personal, family, home, school, and societal barriers is provided, as well as recommendations for ways to help young people become more physically active. Bottom line: Clinicians need to recognize and seek out proven ways to address the barriers that exist regarding inactivity in children and adolescents to get young people on the path to optimal health and fitness (2).